The GEx experiment detected rises in oxygen and carbon dioxide gas in soil samples collected on the Martian surface.

"If we assume these gases were produced during the breakdown of organic material together with hydrogen peroxide solution, we can calculate the masses needed to produce the volume of gas measured," Houtkooper explained.

Houtkooper and his colleague Dirk Schulze-Makuch from Washington State University speculate that an organism based on hydrogen peroxide and water could survive the harsh Martian climate, in which temperatures rarely rise above freezing and can go down to minus 238 degrees Fahrenheit (minus 150 degrees Celsius) at the poles.

The hydrogen peroxide would act like antifreeze for the cell, preventing its insides from crystallizing due to the cold.

Hydrogen peroxide-water solutions also tend to attract more water, so the alien organisms could scavenge water molecules from the Martian atmosphere, the thinking goes.

However, if such creatures were exposed to too much water or atmospheres with high humidity, they could theoretically die through over-hydration.

The researchers think this could account for the anomalous GEx results.

The experiment exposed samples to water vapor, which might have killed any hydrogen peroxide-water microbes died.

The resulting breakdown of their cells would release oxygen and their organic compounds would react with the hydrogen peroxide to release carbon dioxide, water vapor and traces of nitrogen.

Houtkooper thinks the microbes could be detected by NASA's Phoenix lander, which launched on Aug. 4 and will arrive at Mars next May.

While it is rare, terrestrial organisms are known to use hydrogen-peroxide. The bombardier beetle, Brachinus crepitans, uses a 25 percent solution of hydrogen peroxide to shoot steam into the face of pursuing predators.

No one suspects there are any beetles on Mars. But most experts have yet to rule out the possibility of microbial life.

"There does not appear to be any basic reason why hydrogen peroxide could not be used by living systems," Houtkooper said. "While organisms on Earth have found it advantageous to include salt in their intracellular fluids, hydrogen peroxide may have been more suitable for organisms adapting to the cold, dry environment of Mars."

Not likely

But Pace, the University of Colorado microbiologist, thinks there is one very important reason why hydrogen peroxide life is unlikely.

For Pace and many other scientists, the definitive experiment performed by the Viking landers was the gas chromatography mass spectrometry (GC-MS) test, which was capable of identifying substances by their chemical makeup.